In the production of components based on thermal effects, methods in bulk micromechanics are mostly used. For this, expensive etching techniques having a high degree of complexity are typically required, in order to be able to lay bare a thin diaphragm on the front side of the wafer, by etching from the rear through a whole semiconductor wafer, which is used for thermal decoupling from structures mounted on it.
Alternatively, the two methods below may be used:                a) oxidized porous silicon (OxPorSi) and        b) oxidized silicon crosspieces (OXOMM, oxidized surface mechanics).        
With respect to the two alternatives, processes for thermal decoupling are involved that are limited to purely front side processes. The methods have the advantage of a higher mechanical stability of the thermally decoupled region, and thus a higher overall stability of the component.
In the “OxPorSi” method, a relatively thick and poorly heat conducting (0.3 to 1 W/mK) porous silicon layer is generated which may, in addition, also be oxidized. Such layers are used, for example, for the thermal decoupling of sensor structures or actuator structures in thermal components, such as thermal, chemical, fluidic sensors and gas sensors.
A flow sensor is described in German Patent Application No. DE 100 58 009 A1, especially for the analysis of gas flows, that has a supporting body and at least one sensor component that is sensitive to the flow of a medium. The sensor component is separated, from area to area, from the supporting body by a porous silicon region or a porous silicon oxide region.
During the production of porous semiconductors, such as porous silicon, generally, an electrochemical reaction between hydrofluoric acid and silicon is used, during which a sponge-like structure is formed in the silicon. For this, the silicon semiconductor substrate (generally, a silicon wafer) has to be polarized anodically with respect to hydrofluoric acid electrolyte. As a result of the generation of a porous structure, the silicon develops a large internal surface and other chemical and physical properties (such as a lower heat conductivity), than the surrounding bulk silicon. By electrochemical etching of the silicon (anodizing) in, for instance, a mixture of hydrofluoric acid and ethanol, porous silicon may be generated by partial etching proceeding more deeply. For the etching of silicon, defect electrons (holes) are necessary at the interface between the silicon and the electrolyte, which are made available by the flowing current. If the current density is less than a critical current density, holes diffuse to recesses lying in the surface, because of the applied electrical field, and there a preferred etching takes place. In the case of, for example, p-doped silicon, the regions between the recesses are etched laterally up to a minimum thickness, until no more holes can penetrate into these regions because of quantum effects, and the etching process is stopped. In this manner, a sponge-like skeleton structure is created, made of silicon and etched-free pores. During the formation of the skeleton structure, since the etching process takes place only in the area of the tips of the pores, the spongy structure of the silicon already etched is maintained. Along with that too, the size of the pores in the regions already etched remains nearly unchanged. The size of the pores depends on the HF concentration in the hydrofluoric acid, on the doping and on the current density, and can amount to from a few nanometers to a few 10 μm. Likewise, the porosity can be set in a range from ca. 10% to more than 90%.
Various doped substrates can be used for producing porous silicon. Normally, one would use p-doped wafers having different degrees of doping. The pattern within the porous silicon can be determined by the doping.
There are various masking methods for the local production of porous silicon, such as the use of masking layers made of SiXNY.
However, one may also make use of the fact that p-doped and n-doped silicon have greatly different etching behavior. With the conditions under which porous silicon can be generated in p-doped silicon, in n-doped silicon this is not possible, or possible only to a small extent. Therefore, a layer at the surface of the p-doped substrate can be n-redoped for the purpose of determining the sensor element patterns (by ion implantation or diffusion).
For the production of components, the porous silicon is regularly generated locally on a silicon substrate in thicknesses of several to several 100 μm. By an oxidation process at temperatures such as 300 to 500° C., the porous silicon may be stabilized in its structure and its heat conductivity may be further reduced, depending on porosity and crystal size. Oxidized porous silicon is created. Subsequently, in one application, the oxidized porous silicon is closed off using a cover layer, for instance, made of CVD (chemical vapor deposition) materials such as SiXNY. Thereafter, using conventional depositing techniques and patterning techniques, one may build up the active or sensitive elements, such as heaters and/or measuring elements, above the region that has been rendered porous.
In the “OXOMM” method, trenches from several μm to several 100 μm are etched into the silicon by a deep etching process, so that silicon lattices, silicon crosspieces or free-standing silicon columns are created. These are oxidized to a higher valency completely or only partially, in order to reduce their thermal conductivity.
Thereafter, the trenches are regularly closed off by a CVD layer, and the surface is planarized if necessary, in order to apply the active and sensitive elements.